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High & Low Pressure Areas: Horizontal & Vertical Movements

Instructor: Matthew Bergstresser
Weather is part of our daily lives. In this lesson, we will explore what causes stormy weather, dry weather, and the general movement of air called wind.

The Most Difficult Job on the Planet

Can you imagine having the job of predicting the future, knowing that peoples' plans depend on your predictions? Sounds stressful. That's what weather forecasters do every day! They aren't blindly predicting the future weather, but it is still extremely difficult. To come up with a forecast, they have to look at a variety of atmospheric factors. One of those factors is air movement; let's see what causes air to move vertically and horizontally.

Vertical Air Movement

Have you ever been in a swimming pool or in the bathtub and held a rubber ball under the water? What happened to that ball when you let go of it? It rose to the surface. That is because the ball is less dense that the water. Density is how much stuff is in a certain volume. Scientifically stated, it is the ratio of mass to volume, or, it is mass divided by volume (m/v).

The density of air is the main factor that determines whether air rises or sinks. Warm air is less dense that cooler air because the air molecules are spread out more. Think of a hot air balloon. They can rise because air is heated inside the balloon.

A parcel of air is a significant amount air, but not so large that the properties of the air are inconsistent within it. When a warm air parcel rises, it cools in the upper part of the troposphere, which is the layer of the atmosphere where weather occurs. When the air cools, the water vapor in it condenses into water droplets that form clouds. Enough cloud formation can lead to precipitation and even to stormy weather. This is the typical pattern for a low pressure cell. Extreme low pressure systems are hurricanes, and the lowest pressure ever recorded in the eye of a hurricane was 882 millibars (mbars)! To put that in context, standard air pressure is just over 1013 mbars, and low- and high-pressure areas are relative to this value.


Conditions typically associated with low air pressure cells
storm


Pressure is how much force is exerted over a certain area. Think of this analogy. Take two pencils and sharpen one to a fine point. Push both pencils with the same force into a piece of paper. The pencil with the pointed tip punctures the paper, while the flat-ended pencil doesn't. This happens because the force you applied to the pencils is more concentrated at the tip of the pointy pencil compared to the flat-ended pencil.

High pressure is when air is sinking. High pressure cells are typically over 1013 mbars. Sinking air is dry, and it is common for the sky to be cloudless and sunny in high pressure. In other words, high pressure cells lead to outdoor-activity weather!


Clear skies are typical for high air pressure cells
clear


Horizontal Air Movement

When I was growing up, when the wind started to blow, the adults around me would say, ''it must be raining somewhere.'' There might be some truth to that colloquialism. Wind is horizontal air movement and is a symptom of differences in air pressure, and we have already learned that low air pressure generally leads to precipitation.

Let's look at a weather map and tie together high and low air pressure cells. Did you ever wonder what the weather in the United States was like on Friday, November 7, 1913? If so, today is your lucky day!


High and low pressure cells
map


There is a wealth of information on weather maps. Let's interpret the map, starting with the air pressures.


Air pressure values are in the yellow boxes
map2


The pressures on the map are in another unit for pressure, inches of mercury. We can see that the pressures highlighted by yellow boxes decrease from left to right until it passes the low-pressure center, where the pressure rises again. Notice that lines are drawn between points of equal air pressure, which are called isobars. Just as water flows from high elevations to lower elevations, air moves from high pressure to low pressure. This is what causes wind!


Air moves from high pressure to low pressure
map3


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